[Cryptography] Windows certificate update breaks/bricks PCs

[Peter Gutmann]

Microsoft has somehow managed to release a certificate update that (among
other things) disables graphic driver updates, Windows Defender, USB 3.0,
further windows updates, and messes up UAC:

http://www.forbes.com/sites/jasonevangelho/2014/12/13/new-windows-7-patch-is-effectively-malware-disables-graphics-driver-updates-and-windows-defender/

Unfortunately I haven't been able to find any more details on causes, does
anyone know how a certificate update could cause this much trouble?  I'm after
real details here rather than just speculation ("maybe a code-signing cert was
replaced").

Peter.